Thermoplastic polymers, are relatively easy to process into a wide variety of fabricated products. Unfortunately, however, for many of these thermoplastic polymers, the high temperature stability, and their physical properties such as mechanical strength at elevated temperatures, as well as their stability in some commonly-used organic solvents, are less than might be desired. Accordingly, methodology has been previously developed to provide heat-induced crosslinking to convert certain thermoplastic polymers, such as thermoplastic polyurethanes, into thermoset polyurethanes having the desired stability at high temperatures and in the presence of solvents. By way of illustration, U.S. Pat. No. 4,255,552 discloses thermoset polyurethane elastomers obtained by adding organic peroxides to a liquid polyurethane-forming composition prior to reacting the composition to form the polyurethane. The '552 patent teaches that a liquid polyurethane-forming composition containing “unactivated hydrogen peroxide” may be formed in a mold into a desired article and then heated to solidify and thermoset the article, or a composition in solid form such as sheet, crumbs, or granules containing “unactivated hydrogen peroxide” may be provided which are can then be formed into a desired article that is then thermoset by heating the article in a press. The organic peroxides disclosed in the '552 patent are said to have a half-life of greater than one hour at 100 deg C. Unfortunately, these peroxide-containing compositions are less stable than might be desired or necessary for a thermoplastic process such as extrusion or molding. The peroxide containing polyurethane composition would most likely thermoset prior to forming the desired finished article, thus providing technology that is not commercially practical for thermoplastics.
An alternative to heat induced crosslinking of thermoplastics into thermosets by irradiation is known in the art. For example, a technical journal article entitled “Radiation Crosslinked Thermoplastic Polyurethane”, published in the journal International Polymer Science and Technology, Vol. 19, No. 1, pp. T/6-T/9 (1992), discloses the production of a thermoset polyurethane by using a polyisocyanate and methacrylate monomer as a radiation-cross-linkable monomer. This technical journal article does not disclose the particular polyisocyanate used in making polyurethanes disclosed therein. Further, not all polyisocyanates perform alike in irradiation-crosslinking of TPUs.
Another example is U.S. Pat. No. 4,762,884 issued Aug. 9, 1988 for “Process for the Production of Radiation-Crosslinked Thermoplastic Polyurethanes”. This patent discloses the use of a cross-linking agent being a monomeric acrylate or methacrylate.
In view of the above, there is a continuing need in the polymers manufacturing community for new thermoplastic polymer compositions that can be readily thermoset by cross-linking when desired. Such compositions would provide advantageous processing capability, such as by extrusion, when the composition is in the thermoplastic state, and advantageous elevated temperature stability, solvent resistance and other enhanced properties when the composition is thermoset after formation into the desired product. Moreover, new uses for compositions known to be cross-linkable in the presence of irradiation, would also be desirable. The present invention provides such desirable polymer compositions, together with processes for the production of the composition, as well as new uses for compositions known to be irradiation cross-linkable.